Re: [edk2-devel] CPU hotplug using SMM with QEMU+OVMF

Igor Mammedov <imammedo@...>

On Mon, 26 Aug 2019 17:30:43 +0200
Laszlo Ersek <lersek@...> wrote:

On 08/23/19 17:25, Kinney, Michael D wrote:
Hi Jiewen,

If a hot add CPU needs to run any code before the
first SMI, I would recommend is only executes code
from a write protected FLASH range without a stack
and then wait for the first SMI.
"without a stack" looks very risky to me. Even if we manage to implement
the guest code initially, we'll be trapped without a stack, should we
ever need to add more complex stuff there.
Do we need anything complex in relocation handler, though?
From what I'd imagine, minimum handler should
1: get address of TSEG, possibly read it from chipset
2: calculate its new SMBASE offset based on its APIC ID
3: save new SMBASE

For this OVMF use case, is any CPU init required
before the first SMI?
I expressed a preference for that too: "I wish we could simply wake the
new CPU [...] with an SMI".

From Paolo's list of steps are steps (8a) and (8b)
really required?
07b - implies 08b
8b could be trivial hlt loop and we most likely could skip 08a and signaling host CPU steps
but we need INIT/SIPI/SIPI sequence to wake up AP so it could handle pending SMI
before handling SIPI (so behavior would follow SDM).

See again my message linked above -- just after the quoted sentence, I
wrote, "IOW, if we could excise steps 07b, 08a, 08b".

But, I obviously defer to Paolo and Igor on that.

(I do believe we have a dilemma here. In QEMU, we probably prefer to
emulate physical hardware as faithfully as possible. However, we do not
have Cache-As-RAM (nor do we intend to, IIUC). Does that justify other
divergences from physical hardware too, such as waking just by virtue of
an SMI?)
So far we should be able to implement it per spec (at least SDM one),
but we would still need to invent chipset hardware
i.e. like adding to Q35 non exiting SMRAM and means to map/unmap it
to non-SMM address space.
(and I hope we could avoid adding "parked CPU" thingy)

Can the SMI monarch use the Local
APIC to send a directed SMI to the hot added CPU?
The SMI monarch needs to know the APIC ID of the
hot added CPU. Do we also need to handle the case
where multiple CPUs are added at once? I think we
would need to serialize the use of 3000:8000 for the
SMM rebase operation on each hot added CPU.
I agree this would be a huge help.
We can serialize it (for normal hotplug flow) from ACPI handler
in the guest (i.e. non enforced serialization).
The only reason for serialization I see is not to allow
a bunch of new CPU trample over default SMBASE save area
at the same time.

There is a consideration though, an OS level attacker
could send broadcast SMI and INIT-SIPI-SIPI sequences
to rigger race, but I don't see it as a threat since
attack shouldn't be able to exploit anything and in
worst case guest OS would crash (taking in account that
SMIs are privileged, OS attacker has a plenty of other
means to kill itself).

It would be simpler if we can guarantee that only
one CPU can be added or removed at a time and the
complete flow of adding a CPU to SMM and the OS
needs to be completed before another add/remove
event needs to be processed.
I don't know if the QEMU monitor command in question can guarantee this
serialization. I think such a request/response pattern is generally
implementable between QEMU and guest code.

But, AIUI, the "device-add" monitor command is quite generic, and used
for hot-plugging a number of other (non-CPU) device models. I'm unsure
if the pattern in question can be squeezed into "device-add". (It's not
a dedicated command for CPU hotplug.)

... Apologies that I didn't add much information to the thread, just
now. I'd like to keep the discussion going.


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